This invention relates to the field of optoelectronic devices, and more particularly relates to optoelectronic devices that use reflectors such as vertical cavity surface emitting lasers and resonant cavity photodetectors.
Conventional semiconductor lasers have found widespread use in modern technology as the light source of choice for various devices, e.g., communication systems, compact disc players, and so on. For many of these applications, a semiconductor laser is coupled to a semiconductor receiver (e.g. photodiode) through a fiber optic link or even free space. This configuration may provide a high speed communication path.
A typical edge-emitting semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers. The low bandgap layer is termed the "active layer", and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types and when current is passed through the structure, electrons and holes combine in the active layer to generate light.
Another type of semiconductor laser which has come to prominence in the last decade are surface emitting lasers. Several types of surface emitting lasers have been developed. One such laser of special promise is termed a "vertical cavity surface emitting laser" (VCSEL). (See, for example, "Surface-emitting microlasers for photonic switching and interchip connections", Optical Engineering, 29, pp. 210-214, March 1990, for a description of this laser. For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled "Top-emitting Surface Emitting Laser Structures", which is hereby incorporated by reference, and U.S. Pat. No. 5,475,701, issued on Dec. 12, 1995 to Mary K. Hibbs-Brenner, and entitled "Integrated Laser Power Monitor", which is hereby incorporated by reference. Also, see "Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 .mu.m", Electronics Letters, 26, pp. 710-711, May 24, 1990.)
Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry, such as amenability to one- and two-dimensional arrays, wafer-level qualification, and desirable beam characteristics, typically circularly-symmetric low-divergence beams.
VCSELs typically have an active region with bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks which are typically formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is typically turned on and off by varying the current through the active region.
High-yield, high performance VCSELs have been demonstrated, and exploited in commercialization. Top-surface-emitting AlGaAs-based VCSELs are producible in a manner analogous to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields.
VCSELs are expected to provide a performance and cost advantages in fast (e.g. Gbits/s) medium distance (e.g. up to approximately 1000 meters) single or multi-channel data link applications, and numerous optical and/or imaging applications. This results from their inherent geometry, which provides potential low-cost high performance transmitters with flexible and desirable characteristics.
Most VCSELs of practical dimensions are inherently multi (transverse) mode. Single lowest-order mode VCSELs are favored for coupling into single-mode fibers, and are advantageous for free-space and/or wavelength sensitive systems, and may even be beneficial for use in extending the bandwidth-length product of standard 50 .mu.m and 62.5 .mu.m GRIN multi-mode fiber. However, it has long been known that, although the short optical cavity (.congruent.2.lambda.) of the VCSEL favors single longitudinal mode emission, the multi-wavelength (.congruent.10.lambda.) lateral dimensions facilitate multi-transverse mode operation.
Higher order modes typically have a greater lateral concentration of energy away from the center of the lasing cavity. Thus, the most obvious way to force the laser to oscillate in only a lowest order circularly symmetric mode is to make the lateral dimension of the active area small enough to prevent higher-order modes from reaching threshold. However, this necessitates lateral dimensions of less than about 5 .mu.m for typical VCSELs. Such small areas may result in excessive resistance, and push the limits obtainable from conventional fabrication methodologies. This is particularly true for implantation depths of greater than about 1 .mu.m, where lateral straggle may become a limiting factor. Thus, control of transverse modes remains difficult for VCSEL's of practical dimensions.
One suggested approach to help control transverse modes in VCSELs is to use a smaller dimension "w" for an exit window relative to the excitation dimension "g" of the lasing cavity. This approach is suggested in Morgan et al., "Transverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasers", IEEE Phot. Tech. Lett., Vol. 4, No. 4, p 374 (April 1993). Morgan et al. suggest making the lateral dimension of the exit window "w" in a top metal contact smaller than the gain guide aperture "g" (which defines the lateral dimension of the excitation area of the lasing cavity). In this configuration, the top metal contact (typically gold) interfaces directly with the top layer of the top semi-conductor mirror stack, and around the perimeter of the excitation area of the lasing cavity.
In the exit window, the top layer of the top semi-conductor mirror stack interfaces directly with air or the like. Morgan et al. suggest that this configuration may help control lateral mode emission because the reflectivity of the gold-semiconductor interface may be phase-mismatched and/or non-specular, and thus, may provide less reflectivity than the semiconductor-air interface.
The lower reflectivity at the gold-semiconductor interface may reduce the possibility that higher order modes will generate in and around the perimeter of the lasing cavity. This may effectively reduce the lateral optical dimension of the excitation of the lasing cavity, and allow larger fabrication dimensions.
A limitation of this approach is that the discrimination between the reflectivity of the gold-semiconductor interface and the semiconductor-air interface may not be large enough to effectively control higher order modes, particularly at higher bias currents. Further, the reflectance at the gold-semiconductor interface and the semiconductor-air interface may not be sufficiently controllable or reproducible for commercial applications. In addition, the efficiency of these "spatially filtered" VCSELs is non-optimum.
In addition to mode control, another limitation of many prior art VCSEL devices is mode stability. The emission mode emitted by a conventional VCSEL device is often dependent on temperature effects such as thermal lensing and spatial hole burning, bias conditions, etc. Thus, conventional VCSEL devices may not provide a stable output emission mode under all expected operation conditions.
Dielectric resonant reflection filters are discussed in "Theory and Applications of Guided-mode Resonance Filters", S. S. Wang and R. Magnusson, Applied Optics, Vol. 32, No. 14, May 10, 1993. Wang et al. suggest using a dielectric anti-reflective film with a high spatial frequency dielectric grating. Using this approach, it is suggested that a 100% reflective narrow-band spectrally selective mirror may be provided.
The design of narrow band resonant reflection filters based on multilayer waveguide-grating structures is discussed in "Multilayer Waveguide-Grating Filters", S. S. Wang and R. Magnusson, Applied Optics, Vol. 34, No. 14, May 10, 1995). Examples are given for single-, double-, and triple-layer waveguide-grating filters. Here, Wang et al. suggest, among other things, using their narrow band dielectric reflection filters as a mirror for a VCSEL array.
Using a narrow band dielectric resonant reflection filter as a mirror for a VCSEL array has a number of limitations, some of which are described below. First, it is noted that the dielectric reflection filter of Wang et al. is not conductive, and thus cannot be used in the current injection path, for example from a contact of a VCSEL device through a corresponding active region. The mirrors of conventional VCSEL devices typically are semiconductor distributed Bragg reflectors (DBRs). Semiconductor DBR mirrors can be conductive, and often provide a current path from a contact of a VCSEL to the active region. If the dielectric resonant reflection filter of Wang et al. is used to replace, for example, the top mirror of a VCSEL device, the upper contact must be positioned between the dielectric reflection filter and the active region. This may severely limit the design of many VCSEL structures.
Second, the suggested "narrow band" dielectric resonant reflection filter of Wang et al. may not provide a large enough operating wavelength for many opto-electronic applications, including optical communication applications. In "Multilayer Waveguide-Grating Filters", S. S. Wang and R. Magnusson, Applied Optics, Vol. 34, No. 14, May 10, 1995), Wang et al. provide examples of single-, double-, and triple-layer waveguide-grating filters. In each case, the Full Width Half Maximum (FWHM) reflectance bandwidth appears to be less than about 1 nm. A resonance of less than 1 nm may not provide a large enough bandwidth for use in a resonant cavity photodetector (RCPD) or other optical receiver.
A resonant cavity photodetector is typically constructed similar to a VCSEL, but operates in a reverse bias mode. A resonant cavity photodetector may be more efficient than a standard photodiode because the light that enters the cavity, through one of the mirrors, may be effectively reflected through the active region many times. The light may thus be reflected between the mirror stacks until the light is either absorbed by the active region or until it escapes through one of the mirror stacks. Because the mirror stacks are typically highly reflective near resonance, most of the light that enters the cavity is absorbed by the active region.
The "narrow band" dielectric resonant reflection filter of Wang et al. attempts to achieve a very narrow bandwidth, presumably to increase the frequency selectivity of the filter. However, for many VCSEL applications such as in a VCSEL/RCPD electro-optical communication path, it may be beneficial for the wavelength bandwidth of the RCPD device to be increased, and should be broad enough to compensate for a number of factors including manufacturing tolerances of the VCSEL and RCPD devices, device alignment, noise, heat and other factors. This would also be beneficial in utilizing a resonant reflector to construct a multi-mode VCSEL. This may enable a broader bandwidth detector for use in a communication link exhibiting a finite wavelength variation or broad bandwidth source.